The invention relates to a method for the control of a linear drive which has a stator, a rotor which is capable of moving to and fro along a driving axis therein and a drive coil through which a coil current flows, in particular for a linear compressor which has a piston housing and a compressor piston which is capable of moving to and fro along a piston axis therein and is driven by a linear drive, as well as a device comprising a linear drive which has a stator, a rotor which is capable of moving to and fro along a drive axis therein, a drive coil through which a coil current flows and a means for the control of a coil current, in particular for a linear compressor which has the linear drive, a piston housing and a compressor piston which is capable of moving to and fro along a piston axis and can be driven by the linear drive, as well as a method for cooling goods and/or for compressing a fluid.
In a linear compressor, the compressor piston which is capable of moving to and fro along an axis between a first and a second reversal point must be mounted or guided in a direction perpendicular to said axis. Moreover, the kinetic energy of the compressor piston which moves to and fro must be intermediately buffered at the reversal points, i.e. at the points at which the direction of movement of the compressor piston reverses, in order to allow the direction of movement of the compressor piston to be reversed with as little loss as possible. By reversing the direction of movement, the compressor piston implements an oscillating, essentially translational to-and-fro movement in a piston housing. With the help of the to-and-fro movement, a compression process is carried out.
When guiding the oscillating compressor piston, the aim, in order to achieve as high a degree of efficiency of the compressor as possible, is to bring the compressor piston as near as possible to a valve plate of the piston housing in order thus to keep a dead volume formed in front of the valve plate as small as possible. Here however, a collision of the compressor piston with the valve plate must be avoided since a collision of the compressor piston can lead to damage to the compressor piston or to a valve plate. Moreover, a piston rod for the power transmission between compressor piston and linear drive can break, which likewise leads to functional incompetency of the linear compressor. For this reason a safety clearance should always be maintained between the compressor piston and the valve plate.
The object is therefore to develop the armature oscillation as uniformly as possible and with as small a dead volume in front of the valve plate as possible, without the compressor piston striking the valve plate.
Known linear drives frequently require either a precise adaptation of the individual moveable mechanical components to one another, or require a temporary deceleration of the moving parts with the linear drive in order to be able to control the movement in an adequately precise fashion. Fine-tuning the compressor is complicated however, and deceleration with the linear drive reduces its degree of efficiency.
The unexamined German application DE 24 17 443 discloses connecting a coil current of the linear drive via a converter connection (H bridge) in phase with a natural armature oscillation. WO 01/48379 A1 and US 2003/0021693 A1 disclose how to measure time points at which a piston passes through a predetermined position in the vicinity of an upper reversal point. The voltage amplitude on the drive coil is adapted or controlled according to the time measurement, wherein the voltage on the drive coil is predetermined with the aid of an H-bridge. WO 01/71186 A2 and WO 03/081040 A1 describe special position sensors in linear compressors.